Thymosin beta15 is a newly discovered 5300-Da protein that binds actin monomers and inhibits actin polymerization and might thus increase cellular motility. Thymosin beta15 is upregulated at both the mRNA and protein levels in prostate cell lines in a manner directly related to their capacity to metastasize. We hypothesize that because this protein is upregulated in cells with a propensity to metastasize, it might be a useful prognostic marker in breast cancer. Because this is a newly described protein, neither the subcellular localization of thymosin beta15 or its expression in breast cancer has been examined. We describe the use of an affinity-purified polyclonal antibody to show that within breast epithelium, thymosin beta15 is localized diffusely throughout the cytoplasm and that thymosin beta15 is upregulated in malignant (compared with benign) breast tissue. In contrast to the prostate model, thymosin beta15 is upregulated in nonmetastatic breast cancer and even ductal carcinoma in situ (compared with benign breast tissue), and, consequently, it might represent a potential early marker for breast malignancy. Additional studies are needed to evaluate the precise role and prognostic value of thymosin beta15 in breast cancer.